1. Field of the Invention
This invention relates to optical high density recording mediums which comprise a recording layer of at least two organic dyes in which information is optically recorded or written and is subsequently read out from the recorded layer. The invention also relates to a method for making such mediums as mentioned above and to a method for recording optical information in the recording mediums.
2. Description of the Prior Art
The optical recording mediums of the above type may be broadly classified into two groups, one group being a rewritable medium and the other group being a write-once medium. A typical rewritable optical recording medium comprises an optically transparent disc substrate and a recording layer of a photochromic dye formed on the substrate. For information recording, while the optical recording medium is being rotated for tracking, a laser beam having a wavelength of .lambda.1 is spotted on one point of the disc surface. The photochromic dye on the spot undergoes the photochromic reaction and thus, the colorless dye on the spot is colored as having a visible absorption spectrum at .lambda.2. By this, single bit information is recorded on the spot. On reading, a weak laser beam having a wavelength of .lambda.2 is applied to the disc surface. The absorption of the beam in the colored portion can be detected from a change in intensity of transmitted light. On the other hand, when an intense laser beam having a wavelength of .lambda.2 is applied to the colored portion in the same manner as in the writing operation, the photochromic dye in the portion is turned colorless and thus the information can be erased.
If dyes having no photochromic properties are used as the recording layer, and the erasing operation is omitted from the above procedure for the photochromic dye medium, write-once optical recording is effected. However, in view of the principle of the known systems, only one bit is recorded by application of one spot of a laser beam. This places a limitation on the increase of a recorded density.